The ability to systematically manipulate the shapes of inorganic nanocrystal particles remains a goal of modern materials chemistry. The shape and size of inorganic nanocrystal particles control their widely varying electrical and optical properties. One means of achieving shape control is through the use of a static template to enhance the growth rate of one crystallographic face over another. For example, two-dimensional films are obtained when there is favorable epitaxy on a substrate (Cho, J. Cryst., Growth, 202:1-7 (1999)). Pyramidal “dots” are obtained if there is strain between the growing crystallite and the epitaxial substrate, as in the growth of InAs on GaAs (Leon et al., Science, 267:1966-1968 (1995)) and Ge on Si (Liu et al., Phys. Rev. Lett., 84:1958-1961 (2000)).
Anisotropic inorganic nanocrystal particles have also been grown in liquid media. The vapor-liquid-solid growth mechanism in which a solid rod grows out of a supersaturated droplet has been used to create one-dimensional materials (Hu et al., Accounts of Chemical Research, 32:435-445 (1999)), and has been applied to the growth of (insoluble) nanorods in a liquid medium (Trentler et al., Science, 270:1791-1794 (1995); Holmes et al., Science, 287:1471-1473 (2000)).
While anisotropic nanocrystal particles are useful, it would be desirable if nanocrystal particles with other shapes could be formed. As will be explained in further detail below, complex shaped nanocrystal particles such as tetrapods have a number of features that make them more desirable than nanocrystal rods or spheres for some applications. Other advantages of complex shaped nanocrystal particles are described below.